The present invention is directed to methods for increasing the lubricity of a film of amorphous carbon, to amorphous carbon films formed by such methods, and to anodized aluminum surfaces sealed with those films.
Diamond-like carbonxe2x80x94or xe2x80x9camorphous carbonxe2x80x9dxe2x80x94is known to have a low coefficient of friction. As a result, amorphous carbon commonly is used to coat a variety of materials which are exposed to friction and wear during use.
Aluminum is commonly used to manufacture many different articles, some of which are exposed to substantial friction and wear during use. Aluminum tends to resist corrosion because a substantially transparent xe2x80x9cnaturalxe2x80x9d oxide layer forms at the surface of aluminum upon exposure to air. The oxide layer prevents direct contact between the underlying aluminum and corrosive materials in the surrounding environment. Unfortunately, this xe2x80x9cnatural oxidexe2x80x9d layer does not always have a uniform thickness. Because of this, natural oxides generally are removed from aluminum products, and the product then is xe2x80x9canodized,xe2x80x9d or controllably oxidized, to provide a protective oxide layer with better quality and substantially greater thickness.
Anodizing processes generally involve the use of a bath containing an electrolyte, such as sulfuric acid, oxalic acid, chromic acid, phosphoric acid, or combinations thereof, with or without certain addition agents. The aluminum workpiece generally is used as an anode and a component made of steel or other suitable material is used as a cathode. The anode and cathode are immersed in the electrolyte solution, and a direct or alternating current is passed through the electrolyte.
Although anodizing, itself, imparts satisfactory corrosion resistance to aluminum components, anodizing also suffers from several disadvantages. One disadvantage is the porosity of the oxide formed at the surface of the aluminum component. A typical anodizing treatment results in a porous polygonal cellular microstructure superimposed on a thin (less than 100 nm) xe2x80x9cbarrierxe2x80x9d layer. The diameter of the pores in the microstructure can be as small as 10 nm. The cell dimension can be as small as about 30 nm.
The pores formed at the surface of anodized aluminum are undesirable because they tend to serve as corrosion sites, which give rise to deep pits. Deep pits in the anodized surface often result in xe2x80x9cbloomsxe2x80x9d or white spots on the surface of the aluminum. In order to protect anodized aluminum from corrosion, especially in halide or salt-containing environments, the pores of the aluminum oxide customarily are sealed by immersion in a hot solution containing hexavalent chromium. A complex chemical reaction occurs, forming a solid compound of chromium, aluminum, oxygen, and some hydrogen within the pores of the anodized surface. This solid compound seals the pores against penetration by corrosive agents.
Unfortunately, hexavalent chromium solutions are toxic. The use and disposal of hexavalent chromium solutions therefore creates environmental concerns. Environmental concerns, and their associated costs, have created an urgent need for alternative sealing processes that are free from such hazards. The need for alternative sealing processes is intensified because the process used to form a chromate sealant does not purge the pores of the aluminum before or while the chromate sealant is formed. As a result, at least some gas remains in many of the pores, serving as corrosion sites.
Some have attempted to develop alternate sealing processes using other chemical solutions. Until recently, these alternative chemical solutions have not been entirely successful. The present invention provides an effective method for sealing anodized aluminum surfaces with a sealant which is not only non-toxic, but even more lubricious than previous amorphous carbon coatings.
The present invention provides a film of amorphous carbon comprising as an integral component an effective amount of a lubricity-increasing agent. In a preferred embodiment, the film is disposed on and substantially fills the pores in an anodized aluminum surface.